Steam cracker product fractionation

Abstract

The invention generally relates to processes for separating steam cracker products by fractional distillation, and to systems and apparatus useful in such processes. More specifically, the invention relates to decreasing the amount of fractionator fouling that can result from increasing the amount of hydrocarbon molecules in the steam cracker feed having four or fewer carbon atoms.

The invention claimed is: 
     
       1. A process for steam cracking hydrocarbon, the process comprising:
 (a) combining a hydrocarbon feed with steam to produce a steam cracker feed; 
 (b) exposing the steam cracker feed to a temperature ≥400° C. under steam cracking conditions to produce a steam cracker effluent that is at least partially in the vapor phase; 
 (c) cooling the steam cracker effluent to condense a portion of the vapor phase into a liquid phase; 
 (d) separating in at least one fractionator at least a portion of the steam cracker effluent's vapor phase from the liquid phase, the separated vapor phase comprising molecular hydrogen, methane, and C 2+  hydrocarbon; 
 (e) cooling the separated vapor phase to condense a steam cracker naphtha comprising diolefin and C 5+  hydrocarbon; 
 (f) dividing the condensed steam cracker naphtha into first and second steam cracker naphtha streams, and introducing into the at least one fractionator a first reflux stream comprising at least a portion of the first steam cracker naphtha stream; 
 (g) hydroprocessing at least a portion of the second steam cracker naphtha stream under hydroprocessing conditions effective for dimerization of at least a portion of the second steam cracker naphtha stream's hydrocarbon and hydrogenation of at least a portion of the second steam cracker naphtha stream's diolefin to produce a hydroprocessed steam cracker naphtha; and 
 (h) introducing to the at least one fractionator a second reflux stream comprising at least a portion of the hydroprocessed steam cracker naphtha,
 wherein the at least one fractionator comprises a primary fractionator having a stripping region, a rectification region, and a quenching region, the rectification region being located between the stripping region and the quenching region. 
 
 
     
     
       2. The process of  claim 1 , wherein
 the cooled steam cracker effluent is introduced into the stripping region, and 
 at least part of the cooling of step (e) is carried out in the quenching region. 
 
     
     
       3. The process of  claim 1 , further comprising combining the first and second reflux streams to produce a combined reflux stream, wherein the combined reflux stream is introduced into the fractionator. 
     
     
       4. The process of  claim 1 , wherein the hydrocarbon feed comprises ≥25 wt. % of one or more of ethane, propane and butanes. 
     
     
       5. The process of  claim 1 , wherein the steam cracking conditions include a temperature in the range of 400° C. to 900° C., and a pressure ≥0.1 bar, and a residence time in the range of from about 0.01 seconds to 5.0 seconds, and wherein steam cracker feed has a steam: hydrocarbon feed weight ratio in the range of from 0.1 to 1.0. 
     
     
       6. The process of  claim 1 , wherein the hydroprocessing includes catalytic hydroprocessing, and the hydroprocessing conditions include a temperature in the range of from 40° C. to about 370° C., a pressure in the range of about 6 bar (absolute) to about 35 bar (absolute), and a weight hourly space velocity (WHSV) feed rate per weight of catalyst of from about 1 h −1  to about 15 h −1 . 
     
     
       7. The process of  claim 1 , further comprising separating from the hydroprocessed steam cracker naphtha a first hydroprocessed steam cracker naphtha and a second hydroprocessed steam cracker naphtha, wherein
 (i) the separating of the first and second hydroprocessed steam cracker naphthas is carried out before step (h), 
 (ii) the first hydroprocessed steam cracker naphtha comprises C 5  hydrocarbon, and 
 (iii) the second hydroprocessed steam cracker naphtha comprises C 6+  hydrocarbon and includes at least a portion of the hydroprocessed steam cracker naphtha's dimers. 
 
     
     
       8. The process of  claim 7 , further comprising separating from the second hydroprocessed steam cracker naphtha a light hydroprocessed steam cracker naphtha and a heavy hydroprocessed steam cracker naphtha, wherein
 (i) the light hydroprocessed steam cracker naphtha comprises C 6 -C 10  hydrocarbon and includes at least a portion of the hydroprocessed steam cracker naphtha's dimers, 
 (ii) the heavy hydroprocessed steam cracker naphtha comprises C 9 -C 12  hydrocarbon, and 
 (iii) ≥90% of the second reflux stream is the light hydroprocessed steam cracker naphtha. 
 
     
     
       9. The process of  claim 8 , further comprising transferring to the at least one fractionator at least a portion of the heavy hydroprocessed steam cracker naphtha. 
     
     
       10. The process of  claim 1 , wherein the first steam cracker naphtha stream comprises foulant in an amount that decreases during step (h). 
     
     
       11. A method for lessening steam cracker fractionator fouling, the method comprising:
 during a first time interval,
 (a) combining a hydrocarbon feed with steam to produce a steam cracker feed, the hydrocarbon feed comprising C 5+  hydrocarbon; 
 (b) introducing the steam cracker feed into a radiant section of at least one steam cracking furnace and exposing the steam cracker feed in the steam cracking furnace to a temperature ≥400° C. under steam cracking conditions to produce a steam cracker effluent that is at least partially in the vapor phase; 
 (c) cooling the steam cracker effluent to condense a portion of the vapor phase into a liquid phase; 
 (d) separating in at least one fractionator at least a portion of the steam cracker effluent's vapor phase from the liquid phase, the separated vapor phase comprising molecular hydrogen, methane, and C 2+  hydrocarbon; 
 (e) cooling the separated vapor phase to condense a steam cracker naphtha comprising diolefin and C 5+  hydrocarbon; 
 (f) dividing the condensed steam cracker naphtha into first and second steam cracker naphtha streams, and introducing into the at least one fractionator a flow of a first reflux stream comprising at least a portion of the first steam cracker naphtha stream; 
 (g) hydroprocessing at least a portion of the second steam cracker naphtha stream under hydroprocessing conditions effective for dimerization of at least a portion of the second steam cracker naphtha stream's hydrocarbon and hydrogenation of at least a portion of the second steam cracker naphtha stream's diolefin to produce a hydroprocessed steam cracker naphtha; and 
 
 during a second time interval,
 (h) adding one or more of ethane, propane, and butanes to the hydrocarbon feed; and 
 (i) introducing to the at least one fractionator a flow of a second reflux stream comprising at least a portion of the hydroprocessed steam cracker naphtha in order to lessen foulant accumulation in the fractionator,
 wherein the at least one fractionator comprises a primary fractionator having a stripping region, a rectification region, and a quenching region, the rectification region being located between the stripping region and the quenching region. 
 
 
 
     
     
       12. The method of  claim 11 , further comprising combining the flows of the first and second reflux streams to produce a combined flow of the first and second reflux streams, wherein the combined flow of the first and second reflux streams is introduced into the at least one fractionator. 
     
     
       13. The method of  claim 11 , wherein during the first time interval
 (i) the hydrocarbon feed comprises ≥50 wt. % of C 5+  hydrocarbon, 
 (ii) the steam cracker feed has a steam: hydrocarbon feed weight ratio in the range of from 0.2 to 1, and 
 (iii) the steam cracking conditions include a temperature in the range of 760° C. to 880° C., a pressure in the range of from 1.0 bar (absolute) to 5.0 bar (absolute), and a residence time in the range of from 0.10 to 2.0 seconds. 
 
     
     
       14. The method of  claim 11 , wherein during the second time interval
 (i) the hydrocarbon feed comprises >50 wt. % of one or more of ethane, propane, and butanes, 
 (ii) the steam cracker feed has a steam: hydrocarbon feed weight ratio in the range of from 0.2 to 0.5, and 
 (iii) the steam cracking conditions include a temperature in the range of about 760° C. to 1100° C.; a pressure in the range of from 1.0 bar (absolute) to 5.0 bar (absolute), and a residence time in the range of from 0.10 to 2.0 seconds. 
 
     
     
       15. The method of  claim 11 , wherein the hydroprocessing includes catalytic hydroprocessing, and the hydroprocessing conditions include a temperature in the range of from 40° C. to about 370° C., a pressure in the range of about 6 bar (absolute) to about 35 bar (absolute), and a weight hourly space velocity (WHSV) feed rate per weight of catalyst of from about 1 h −1  to about 15 h −1 . 
     
     
       16. The method of  claim 11 , wherein the hydrocarbon feed in the steam cracker feed is introduced into the steam cracker in a first amount during the first time interval and a second amount during the second time interval, the second amount being within +/−10% of the first amount on a weight basis. 
     
     
       17. The method of  claim 12 , wherein the hydroprocessed steam cracker naphtha constitutes ≥10% of the combined flow of the first and second reflux streams. 
     
     
       18. The method of  claim 17 , further comprising separating from the hydroprocessed steam cracker naphtha a first hydroprocessed steam cracker naphtha and a second hydroprocessed steam cracker naphtha, wherein
 (i) the separating of the first and second hydroprocessed steam cracker naphthas is carried out before the transferring of the second reflux stream to the at least one fractionator, 
 (ii) the first hydroprocessed steam cracker naphtha comprises C 5  hydrocarbon, and 
 (iii) the second hydroprocessed steam cracker naphtha comprises C 6+  hydrocarbon and includes at least a portion of the hydroprocessed steam cracker naphtha's dimers. 
 
     
     
       19. The method of  claim 18 , further comprising separating from the second hydroprocessed steam cracker naphtha a light hydroprocessed steam cracker naphtha and a heavy hydroprocessed steam cracker naphtha, wherein
 (i) the light hydroprocessed steam cracker naphtha comprises C 6 -C 10  hydrocarbon and includes at least a portion of the hydroprocessed steam cracker naphtha's dimers, 
 (ii) the heavy hydroprocessed steam cracker naphtha comprises C 9 -C 12  hydrocarbon, and 
 (iii) the light hydroprocessed steam cracker naphtha constitutes ≥90% of the combined flow of the first and second reflux streams. 
 
     
     
       20. The method of  claim 19 , wherein during the second time interval after a predetermined foulant amount in the first steam cracker naphtha stream has been achieved, the flow of the second reflux stream is decreased and the flow of first reflux stream is increased.